This invention relates to an injection powder used in refining steel, and particularly to a lime-based injection powder consisting of carbon dioxide treated CaO, CaF.sub.2 and Al.
It is known that sulphur and oxygen inclusions in steels affect adversely steel refining processes and the mechanical properties of steel such as low temperature toughness, drawability, etc. Generally, in order to improve the quality of steel, the sulphur and oxygen inclusions in steel must be eliminated as completely as possible and the inclusion morphology must be controlled. Lime-based injection powders are commonly used in the manufacture of steel so as to improve the cleanliness, the inclusion morphology and the mechanical properties of the steel.
Quick lime is a known desulphurizing substance which has strong hygroscopic properties and thus is readily reacts with the atmospheric moisture. The moist quick lime has the following disadvantages: (1) the flowability of the powder is poor and thus difficulties are created in the injection operation, for instance, severe blockade is caused in the pipes used for injection; (2) the moisture content contained in the quick lime decomposes readily according to the following equation: EQU H.sub.2 O.fwdarw.2H+O
When the moist quick lime is introduced into a molten steel, it increases the amount of H and O in the steel and thus affects adversely the quality of the steel; (3) The increased oxygen content lowers the efficiency of the desulphurization of the injection powder. To prevent the quick lime from absorbing moisture, the quick lime may be used immediately after calcination or reheated in a drying furnace at a sufficiently high temperature before injection. However, it is practically difficult in most case due to the problems in necessary equipment. Therefore, it is desirable to provide a lime-based injection powder which does not absorb moisture for a prolonged storage period, particularly in an area of high humidity.